1. Field of the Invention
The present invention relates to a method of treating an etching waste fluid and, more particularly, to a method of regenerating a waste fluid produced when nickel or an iron alloy containing nickel such as invariable steel (Invar) is etched with an aqueous solution containing FeCl.sub.3.
2. Description of the Related Art
In recent years, along with developments of televisions, OA equipment, and computers, demand has arisen for a high-precision, high-quality CRT. A high nickel alloy such as Invar has been used as a material of CRT shadow masks. In etching of a shadow mask material consisting of such an alloy, or pure nickel, an aqueous solution containing high-concentration FeCl.sub.3 is used as an etching solution since it allows a moderate and reliable reaction and is free from generation of gases.
During etching using the aqueous FeCl.sub.3 solution, when a metal such as nickel and iron constituting a shadow mask material is partially dissolved, FeCl.sub.3 is reduced into FeCl.sub.2. Meanwhile, iron and nickel are dissolved in the aqueous FeCl.sub.3 solution, into FeCl.sub.2 and NiCl.sub.2, respectively.
FeCl.sub.2 produced in the etching solution is oxidized using chlorine gas, or H.sub.2 O.sub.2 in the presence of hydrochloric acid and is easily converted into FeCl.sub.3. In the course of continued operation of this method, the content of NiCl.sub.2 is increased in the etching system, and eventually the solution cannot be used in practice in view of the reaction rate and chemical equilibrium. In order to circularly use the etching solution, a part of the etching solution is removed as an etching waste fluid, the nickel component is removed from the fluid, and the regenerated solution is returned to the etching system.
Various means are proposed as methods of eliminating nickel from such an etching waste fluid. Those are,
(a) a method of electrolyzing a waste fluid to perform cathodic reduction, thereby precipitating metallic nickel (Published Unexamined Japanese Patent Publication No. 59-31868),
(b) a method of precipitating and separating nickel as a complex by using a complexing agent such as glyoxime having selectivity for nickel (Published Unexamined Japanese Patent Publication No. 59-190367),
(c) a method of substituting Cl.sup.- and precipitating nickel using metallic iron and oxidizing Fe.sup.2+ into Fe.sup.3+ using chlorine (Published Examined Japanese Patent Publication No. 61-44814),
(d) a method of cooling an etching waste fluid after concentration by heating to eliminate an FeCl.sub.2.4H.sub.2 O crystal, firstly supplying HCl gas while cooling the mother liquor to 5.degree. to -10.degree. C. to recover only nickel in the form of an NiCl.sub.2 crystal, and stripping HCl from the treated solution, thereby recovering the treated solution as an FeCl.sub.3 concentrate, and at the same time the stripped and recovered HCl is recycled to the cooling and crystallization step (Published Examined Japanese Patent Publication No. 63-10097), and
(e) a method of absorbing HCl gas in an etching waste fluid and crystallizing and separating both NiCl.sub.2 and FeCl.sub.2, heating and distilling the mother liquor to partially remove HCl gas and water, adding water and iron pieces to the residual solution to neutralize it, and oxidizing the solution with Cl.sub.2 (Published Unexamined Japanese Patent Publication No. 62-222088).
There is also proposed a method of extractively distilling the recovered hydrochloric acid using FeCl.sub.3 as an extracting medium, thereby extracting high-concentration HCl (Published Examined Japanese Patent Publication No. 63-10097).
In method (a) of all the conventional methods described above, standard precipitation electrode potentials of Fe.sup.2+ and Ni.sup.2+ are close to each other, and nickel tends to cause generation of an overvoltage. It is difficult to selectively reduce and precipitate only nickel. In addition, Fe.sup.3+ is reduced to result in an economical disadvantage. Although method (b) has a high nickel elimination rate, the complexing agent is expensive. Since nickel generally need not be perfectly eliminated, a high nickel elimination rate does not mean a prominent merit. In method (c), since nickel is not precipitated until Fe.sup.3+ is entirely reduced into Fe.sup.2+, a large amount of FeCl.sub.2 is produced. A large amount of Cl.sub.2 is required to oxide the large amount of FeCl.sub.2. Therefore, method (c) is not necessarily a good method of recovering FeCl.sub.3. Although method (d) is one of the most preferable methods, the etching waste fluid must be cooled to a temperature falling within the range of 5.degree. to -10.degree. C., and power cost for cooling is increased. In addition, the treated solution is recovered as an aqueous FeCl.sub.3 solution by simple distillation at atmospheric pressure alone. According to the experiences of the present inventors, it is difficult to sufficiently remove hydrochloric acid in the etching solution to be regenerated and circulated by only such a simple atmospheric distillation alone. When the etching solution contains free hydrogen chloride in an amount exceeding a predetermined limit, hydrogen is produced upon etching. From this point of view and the like, precise and stable operations may be interfered, and a safety problem may be posed. When high-precision etching is required as in etching of a CRT shadow mask, a large amount of metallic iron or iron oxide must be charged into the recovered iron chloride solution as in method (e), in order to neutralize the free hydrochloric acid.
In the neutralization method using the iron component, iron reacts with HCl to produce dangerous hydrogen and at the same time reacts with FeCl.sub.3. Thus, the amount of Fe.sup.2+ is undesirably increased. In order to recover an etching Fe.sup.3+ component, consumption of an oxidant is increased too much. Examples of an easily obtainable iron oxide used for neutralizing HCl are Fe.sub.3 O.sub.4 and Fe.sub.2 O.sub.3. When the former example is taken into consideration as a complex oxide of FeO.Fe.sub.2 O.sub.3, the FeO component is relatively easy to be dissolved. The Fe.sub.2 O.sub.3 component including the latter example as well is difficulty soluble with HCl, thus posing a problem. Problems to be solved are to explore a first method capable of easily dissolving an iron oxide even if HCl having a relatively low concentration is used and a second method of decreasing the HCl concentration in the aqueous FeCl.sub.3 solution containing HCl after nickel elimination from the etching waste fluid without producing a large amount of FeCl.sub.2 as an application of the first method.
In the method of crystallizing NiCl.sub.2 upon absorption of HCl, a water-containing NiCl.sub.2 crystal, a coprecipitated FeCl.sub.2 crystal, or a sludge containing a corrosive material such as FeCl.sub.3 contained in the mother liquor in a high concentration is produced. It is difficult to treat these products. In addition, there is no effective process for systematically recovering HCl having a high concentration. The extractive distillation using FeCl.sub.3 and described in Published Examined Japanese Patent Publication No. 63-10093 does not provide an important effect as expected on the vapor-liquid equilibrium. The extractive distillation with FeCl.sub.3 itself is unstable, and a precipitate which is assumed to be an iron oxide tends to be produced. Therefore, it is difficult to use this extractive distillation.